Radiotherapy is a form of therapy based on the application of radiation to a tumor in order to destroy malignant cells, thereby impeding their growth and reproduction. This therapy may also be applied to normal tissues; however, since tumoral tissues are more sensible to radiation and cannot repair the damage as efficiently as normal tissue, they are destroyed, thus blocking the cellular cycle.
Radiation penetrates a certain distance through the patient's body, in such a way that most of its effect takes place a predetermined distance under the patient's skin depending on the penetrability of each specific radiation type, the effect falling exponentially from that depth. However, sometimes the tumor lies at a depth that is lower than the depth where radiation effect is maximum, such as for example on the patient's skin or very close to it. In those cases, conventional use of radiation hardly has any effect in the tumor area.
In order to solve this drawback, a layer of material having a radiation absorption capability similar to that of the human body may be provided between the radiation source and the skin. If the thickness of the layer is calculated correctly, the radiation acts on the patient's skin. Such an element is known as a “bolus”. Some known bolus are disclosed in the following documents:                U.S. Pat. No. 6,231,858 describes a bolus for radiotherapy made by mixing water with an organic polymer such as gelatin, pectin or various types of gums.        International application WO 2008/119905 describes a bolus for radiotherapy made of polyurethane.        International application WO 2008/054823 describes a bolus for radiotherapy made by a mix of an oil gel and a thermoplastic polymer.        
Nowadays, it is widely considered that any material used for making a bolus must, in addition to being adaptable to the shape of the patient's skin, have a similar density to that of the human body, that is, to that of water. Thus, if a radiation absorption capability similar to that of the human body is achieved, the bolus will respond to radiation in the same way as the body of the patient.
In connection to this, reference can be made, for example, to the above mentioned prior art U.S. Pat. No. 6,231,858, where column 1, line 57 describes being “equivalent to human body tissue”, in the sense, explained some lines below, that “its properties as to radiation absorption and dispersion must be equal to those of human tissue”, as the main feature of a bolus. Also page 3, lines 18-19 of prior art international application WO 2008/054823 disclose that a bolus must have “absorption properties similar to those of tissue (or water)”. Further, page 9, lines 13-19 of this application disclose the addition of filler material specifically to adjust the bolus density to 1 gr/cm3 (the density of water). The third mentioned international application, WO 2008/119905, emphasizes the fact that the bolus disclosed in this document has a density close to 1 (see page 6, lines 30-32 or page 10, lines 1-8) as an advantageous feature of the invention. Further, page 2, line 28 of the same document discloses that fact that silicone has a density different from 1 as a specific drawback of using silicone.
Furthermore, specifically silicone is considered a material not suitable for making a bolus due to other reasons in addition to its density. For example, column 2, lines 16-29 of international application WO 2008/054823 mentions drawbacks related with cost and cleaning problems in connection with a silicone bolus. Page 6, lines 30-32 of international application WO 2008/119905 also discloses some drawbacks of using silicone for making a bolus, such as the high cost and handling difficulties.